Need help manually finding the standard deviation using an external function

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Kelley Kehoe 2020-10-2

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评论： Kelley Kehoe 2020-10-3

I am new to matlab (taking an intro to matlab class), but for my assignment I have to do a number of things including randomly generating 10 scores for each of 25 students. Then take the median, mean, and standard deviation for each student's scores. For median and mean I used median(Score,2) and mean(Score,2) to find the values for each row (individual student's 10 scores). I am struggling to figure out how to manually find the standard deviation for each student. I also have to use Matlab functions to find the std and compare it to the manual calculation. The part I am inquiring about is part D in the code. I know I probably dont have it right at all this is just my starting point from looking stuff up online. Any help would be appreciated, thanks.

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Answer 1

Ameer Hamza 2020-10-2

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编辑：Ameer Hamza 2020-10-2

在 MATLAB Online 中打开

It should be something like this

z = sqrt(1/N*sum((Score - mean_student).^2, 2))

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Ameer Hamza 2020-10-2

MATLAB uses following formula for std

The difference is division by N-1 in this case. In your formula, there is a division by N. This is an unbiased estimator of standard deviation. Your formula is a biased estimator. Both are equally valid, and you should use the one given to you. Read the discussion here: https://en.wikipedia.org/wiki/Standard_deviation#Estimation.

You cannot make the MATLAB's std result match your formula.

Kelley Kehoe 2020-10-3

Thanks! For a later part of the assignment I have to use that same z function but the input is the vector of means for each student, I am supposed to get an output of a single number but I am getting a 25x1 vector output similiar to the first function. Any thoughts on this?

This is my attempt:

formula_std_class_mean = sqrt(1/N*sum((mean(Score,2) - class_mean_grade).^2, 2)

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